Not necessarily 100% of the time would you require multiple video cards. Matrox' TripleHead2Go can divide the output signal from one video card onto three monitors. All you need is a video card with a dual-link DVI interface. Naturally, there are some perspective and POV idiosyncrasies associated with using multiple monitors for three-dimensional games and renders.

My understanding is that the future DisplayPort uses a single, multi-channel cable that can daisy-chain multiple displays.